It is known to use, in converter equipment, so-called asymmetrically semi-controlled single-phase converter bridges which, between the d.c. connections, have two thyristor branches connected in series with each other, and in parallel with two diode branches connected in series with each other. Such a converter normally has a lower consumption of reactive power than a conventionally controlled single-phase bridge having thyristors in all four branches. The consumption of reactive power is, however, still high, and often unacceptably high when the bridge voltage is controlled to a fraction of its maximum value.
It is also known that an additional reduction of the consumption of reactive power (i.e. an increase of the power factor) may be achieved by series-connecting two or more bridges, for example of the above-mentioned kind, in a converter, and by controlling the converter so that always at the most one bridge works with a variable control angle while the other bridges are controlled either to maximum or minimum direct voltage. However, converters of this kind require a relatively large number of semiconductor components (thyristors and diodes), and therefore they are complicated, bulky and expensive.
British patent specification No. 1,497,045 discloses converter equipment in which a converter consists of two series-connected asymmetrically semi-controlled bridges. To achieve an even load distribution between the secondary windings of the converter transformer, while at the same time obtaining a low consumption of reactive power, each of the two bridges is controlled cyclically, the cycles for the two bridges being displaced in time with respect to each other. The length of the cycle is one or two full periods. During each of the half-periods of the cycle, one of the bridges is always working with a variable control angle, that is, the control angle generally assumes a value which deviates from the values (0.degree. and 180.degree.) for which the harmonic content of the mains alternating current and the consumption of reactive power are lowest. The consumption of reactive power and the harmonic content of the alternating current are therefore relatively high. Furthermore, this known equipment requires that each converter should comprise at least two d.c. series-connected bridges, which makes the equipment relatively complicated and expensive.
The present invention aims primarily to provide converter equipment in which it is possible to achieve--with a simple converter, for example with a single converter bridge--a lower consumption of reactive power and a lower harmonic content of the main alternating current supply than has hitherto been possible with equipment of a corresponding degree of complexity.
A subsidiary aim of the invention is to provide equipment with one or more simple converters in which the d.c. component in the alternating current of each individual converter is zero. This makes it possible to completely eliminate the direct voltage component in the alternating current of the equipment, both in equipment having a single converter and in equipment having several converters with different load objects.